1. Field of the Invention
The present invention generally relates to medical devices and more particularly relates to medical devices used to monitor patient respiration.
2. Description of the Prior Art
It has been known for some time that it is desirable to monitor patients to provide an alarm to attending personnel upon cessation of or the presence of grossly irregular respiration. Often monitoring devices are employed during the sleep of patients who are prone to central sleep apnea. Many of these patients are children or infants. An alarm condition indicates that remedial action is quickly needed to restore normal respiration before the patient suffers irreversible damage or death as a result of oxygen deficiency.
The early monitoring devices employed a number of sensor types to determine abnormal respiration. Sensors to measure airflow, blood gas composition, and stretching of the thorax have all been used with some degree of success. However, the most widely used monitoring technique in current clinical use measures changes of impedance across the thorax of a patient. This is accomplished by passing a small current through the thorax between two surface electrodes. As a patient inhales, the lungs fill with air, the thorax becomes less conductive, and the measured resistance across the thorax increases. Upon exhaling, the reverse happens, and the resistance decreases.
In the most practical systems, this small current is an alternating current having a frequency of 25-100 khz. By using this frequency range, the electrical circuitry can quite readily reject a number of noise components located on either side of the basic carrier frequency.
Through use of various signal processing techniques, considerable noise rejection is obtained. However, problems continue to exist with artifacts at or near the respiration rate. U.S. Pat. No 4,803,997, issued to Bowman and incorporated herein by reference, addresses the presence of an artifactual respiratory signal, which recurs at the basic heart rate of the patient, often referred to as cardiac artifact. In Bowman the cardiac artifact signal is explicitly sensed and processed using separate detection circuitry to ensure that the cardiac artifact is not treated as a respiration signal.
Rejection of the cardiac artifact using the Bowman invention provides a significant improvement. However, the cardiac artifact is periodic and recurring. Thus, that circuit is not completely effective in addressing artifacts caused by aperiodic and transient events such as motion of the patient. Such transient artifacts can cause swamping of the electronic circuitry, false indications of respiration, or both.